Github App: Permission on “single file” not working

I created a Github App under my Organization and I set it as public so the users of my Organization can install and I used it.

Why I did that? Because the following:

I built a NodeJS backend that uses this Github App to detect changes of an index.html file in the repository and this backend do some stuff to create a new processed file and this file is being uploaded to the same repository under build/index.html.

But the users of my application can fork the repositories created in the Organization to update the index.html in their forked repository and this NodeJS backend will update the build/index.html in their repository.

This works as expected If I edit the file with the owner’s organization account but is not working for the forked repositories because this permission to create/edit a file https://developer.github.com/v3/repos/contents/#create-or-update-a-file but only for the owner’s organization account and I saw a permission listed in the Github App Settings where you can set a permission on a single file

enter image description here

But is not working and is returning the same error every time. It works to get the file but not to update it.

    {
    “message”: “Resource not accessible by integration”,
    “documentation_url”: “https://developer.github.com/v3/repos/contents/#create-or-update-a-file
    }